Last year it was just a technical term known mainly by the oil industry. This year "dilbit" was at the center of the fight over the Keystone XL pipeline.

This was the year "dilbit" registered on the U.S. political and public radar in a major way.

Previously just a technical industry term, dilbit—short for diluted bitumen—became the center of citizens' concerns about the controversial Keystone XL oil pipeline.

Bitumen is a heavy crude oil mined from Canada's oil sands region. The bitumen is so thick that it must be diluted with liquid chemicals before it can flow through pipelines—hence the term dilbit.

Environmental groups have long opposed oil sands production due to its climate change impacts. But dilbit is also significant because it doesn't behave like conventional crude oil when it spills into water. Those risks became apparent in July 2010, when a ruptured pipeline spewed more than a million gallons of dilbit into Michigan's Kalamazoo River.

A seven-month investigation by InsideClimate News revealed that as the light chemicals in the dilbit evaporated, the bitumen sank to the bottom of the Kalamazoo River—leaving a mess unlike anything regulators and emergency responders had seen before. The investigation also showed how dilbit's unknown health effects had officials scrambling for answers. Enbridge, the pipeline's owner, is still working to dredge bitumen from the river. The total cost of cleanup has exceeded $800 million, making it the most expensive oil pipeline spill in U.S. history.

The InsideClimate News series, called "The Dilbit Disaster," gained national attention for exposing how pipeline and environmental regulators were unprepared for the unique risks posed by dilbit. In the words of one EPA veteran, the "submerged oil is what makes this thing more unique than even the Gulf of Mexico [BP] situation. ...This experience was brand new for us. It would have been brand new for anyone in the United States."

Dilbit is what would be carried in the Keystone XL pipeline, which is slated to pass through the critically important Ogallala aquifer. The industry plans to build or repurpose 10,000 miles of pipelines within the next few years to increase oil sands imports.

The concern of many landowners and activists is that those new pipelines won't require additional safeguards, because dilbit pipelines aren't regulated any differently from conventional oil pipelines.

Notably, however, the regulations do differ when it comes to taxes: Refineries that import conventional crude oil pay an eight-cent per-barrel excise tax that funds a federal spill cleanup fund. Shipments of dilbit are exempt from the tax, despite the fact that the fund was used to cover cleanup costs of the Kalamazoo spill. The exemption exists because Congress didn't consider oil sands to be crude oil when it created the tax in 1980.

Environmentalists say dilbit corrodes pipelines more easily than conventional crude. The National Academy of Sciences is currently reviewing existing research to determine whether that's true—but there are no existing studies on how dilbit and conventional oil behave differently post-spill. The report will be delivered to Congress next July. If the scientists conclude that dilbit is indeed more corrosive, they will conduct a second phase of the study to see if current regulations are sufficient to mitigate that risk.

This month, The Los Angeles Times wrote an editorial calling on the Obama administration to include analysis of "any heightened risks posed by transporting dilbit" in its review of the Keystone XL. It came after a Texas landowner along the right of way sued TransCanada, the company behind the pipeline, for telling him the pipeline would carry only crude oil, not dilbit.

Amid the mounting concerns and scrutiny, TransCanada published its own primer on dilbit, which does not address how bitumen sinks after a spill.